The present invention relates to solid composite electrolytes for use in rechargeable lithium batteries, and more particularly, to a polymer-ceramic composite and ceramic-ceramic composite which exhibit enhanced low-temperature ionic conductivity and low temperature dependence of conductivity.
Widespread interest has existed in the use of solid electrolytes for use in lithium batteries and other high-energy-density power sources. Various classes of materials have been proposed for use including polymers, ceramics, and polymer-ceramic composites, particularly polymer electrolytes. Solid polymer electrolytes generally consist of a high molecular weight polymer such as polyethylene oxide complexed with a lithium salt. However, the conductivity of such electrolytes has been marginal for low (ambient) temperature applications. In addition, such electrolytes possess a low cationic transport number, they exhibit poor interfacial stability with lithium electrodes, and they have a very high activation energy (high temperature dependence) for lithium ion conduction at low temperatures.
Polymer-ceramic composite electrolytes are a known sub-class of solid polymer electrolytes which are formed by incorporating a ceramic material in the polymer matrix to enhance conductivity. For example, ceramic additives such as Al.sub.2 O.sub.3, LiAlO.sub.2, SiO.sub.2, and zeolite have been used in small amounts to increase the room temperature conductivity of composite electrolytes. See Capuano et al. "Composite Polymer Electrolytes", J. Electrochem. Soc. 138, 1918 (1991) which teaches the incorporation of .gamma.-Al.sub.2 O.sub.3 and LiAlO.sub.2 in a poly(ethylene oxide) polymer.
The use of lithium nitride (Li.sub.3 N) has also been proposed for use in composites as it has an high ionic conductivity at ambient temperatures of the order of about 10.sup.-3 S cm.sup.-1. See Skaarup et al. "Mixed Phase Solid Electrolytes", Solid State Ionics, 28-30, 975 (1988), which teaches a polymer composite containing Li.sub.3 N. See also commonly assigned U.S. Pat. No. 5,695,873, which teaches a polymer-ceramic composite electrolyte containing lithium nitride. However, while the incorporation of ceramic materials in composite electrolytes results in increased conductivity as compared with solid polymer electrolytes, up until now such increases have been relatively marginal, even when such electrolytes have been subjected to low temperature annealing. In addition, the processing of such polymer-ceramic composite electrolytes in thin film applications has been limited due to the relatively large volume fraction of lithium nitride used as well as the brittleness of lithium nitride.
Accordingly, there is still a need in the art for a solid composite electrolyte for use in lithium batteries and other electrochemical applications which may be easily manufactured and which exhibits high conductivity at ambient temperatures and low temperature dependence of conductivity.